Schematic cross section of the San Andreas Fault Zone at Parkfield, CA, showing the drill hole for the San Andreas Fault Observatory at Depth (SAFOD) started in June 2004 and the pilot hole drilled in 2002.   Red dots in the drill holes show sites of monitoring instruments.   White dots represent locations of minor seismicity (prior to the 2004 earthquake) at depths
of 2.5 to more than 10 km.  The subsurface colors show the electrical resistivity of rocks as determined from surface surveys; the lowest-resistivity rocks (red) above the area of minor earthquakes may represent a fluid-rich zone that reflects the presence of highly fractured rocks.